Magnetic resonance imaging (MRI) is a technique using magnetic resonance for imaging. The principle of magnetic resonance includes nuclei containing protons (e.g., the proton of a hydrogen nucleus) widely existing in the human body that have a spinning movement (e.g., like a small magnet), and the spin axes of these small magnets do not have a certain rule. If an external magnetic field is applied, these small magnets will be rearranged according to the magnetic force lines of the external magnetic field, and arranged in two directions that are parallel or antiparallel to the magnetic force lines of the external magnetic field. The direction parallel to the magnetic force lines of the external magnetic field is called a positive longitudinal axis, and the direction antiparallel to the magnetic force lines of the external magnetic field is called a negative longitudinal axis. The nuclei only have a longitudinal magnetization component, and the longitudinal magnetization component has both direction and amplitude. Nuclei in the external magnetic field are excited by a radio-frequency (RF) pulse with a specific frequency to make the spin axes of these nuclei deviate from the positive longitudinal axis or the negative longitudinal axis to produce resonance. This is magnetic resonance. After the spin axes of the excited nuclei deviate from the positive longitudinal axis or the negative longitudinal axis, the nuclei have a transverse magnetization component. After stopping transmitting radio-frequency pulses, the excited nuclei transmit echo signals to release the absorbed energy step by step in the form of electromagnetic waves. The phase and energy level thereof both recover to the state before being excited, and the image may be reconstructed after the echo signals transmitted by the nuclei are subjected to further processing such as space encoding.
The magnetic resonance imaging (MRI) system includes various coils such as, for example, a body coil covering the whole body area and a local coil only covering some part of the body. A local coil having a receiving antenna is widely used in the magnetic resonance imaging system. The local coil is applicable to different body parts of different sizes with a good signal to noise ratio. The local coil may have multiple functions. For example, flexible coils manufactured by Siemens may be used for magnetic resonance imaging of all parts of a human body, such as, for example, chest/abdomen/elbow/knee/ankle/head.
Inside the local coil is a receiving antenna, and outside the local coil is a layer of soft material, so that the local coil has the feature of flexibility. However, the receiving antenna inevitably has rigid components or large components such as, for example, diodes, sensors, and capacitors. For example, an active detuning circuit and a passive detuning circuit of the receiving antenna use rigid elements such as a printed circuit board (PCB). In order to protect the rigid elements, a plastic box may be used as a rigid housing. The abovementioned method is often used in places such as a body array coil and a flexible coil of a Siemens magnetic resonance imaging system.
However, the rigid housing has the following defects while protecting the rigid elements.
1. The flexibility of the receiving antenna is reduced, and the application of the local coil is limited. For a mini size coil, the bending angle of the antenna is very small.
2. Since the rigid components and the flexible parts are connected, the antenna is easily damaged. This is true for the connection positions of the rigid components and the flexible parts under long-time bending.
3. The size of the receiving antenna may not be made sufficiently small due to the adoption of the rigid housing.
In addition, due to the error in the capacitance value of the PCB and the manufacturing error in the copper plating structure of the antenna, an adjustable capacitor is to be adopted to make the frequency of the local coil match the operating frequency of the system. However, the adjustable capacitor may be larger in size and is difficult to be used in the flexible coil. In addition, after the rigid part of the local coil is covered by the flexible material, the frequency of the local coil may be unadjustable. The practice in the prior art is to adjust the capacitor to the operating frequency of the system before covering the rigid part with the flexible material. In many cases, after the flexible material is covered, the frequency of the local coil still does not match the operating frequency of the system, so that the local coil may only be scrapped.